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Diamond-trap experiments in combination with laser ablation ICP-MS analyses of frozen samples have been used to directly determine the composition of fluids and melts in equilibrium with crust and mantle mineral assemblages. Here we: (1) describe an improved freezing cell that utilizes electronic cooling elements to facilitate routine laser ablation measurement on diamond traps and (2) demonstrate that the major element stoichiometry of subsolidus aqueous fluids in equilibrium with crystalline mineral assemblages can be measured using the diamond-trap method with comparable precision and accuracy to single-crystal weight-loss experiments.

The importance of magma degassing and separation of magmatic fluids (aqueous as well as other components) has been recognized in many geologic observations at least since Niggli (1912; see also Morey and Niggli 1913; Bowen 1928; Morey 1957). Improved analytical techniques in recent years have permitted micron scale examination of melt and fluid inclusions inside magmatic minerals. In these inclusions, concentrated aqueous solutions and extreme enrichment of melts and fluids in volatile components (S, CO2, B2O3, Cl, F, and metals) have been observed (Roedder 1992; Webster 1997; Thomas et al. 2000; Kamenetsky et al. 2004). Pegmatite formation reflects the role of exsolution of aqueous fluid from silicate melt (Jahns and Burnham 1969) at the magmatic–hydrothermal transition (e.g., Veksler 2004). Fluid–gas immiscibility is clearly involved in formation of pegmatites, and for some other fluids originating in magmatic or metamorphic environments. Certain natural fluid systems rich in volatile components are immiscible and others show completely mixed compositions at geological temperatures and pressures (e.g., Roedder 1984; hydrocarbons, Pedersen and Christensen 2007; geothermal and submarine hydrothermal systems, Arnorsson et al. 2007, Foustoukos and Seyfried 2007; ore-forming fluids, Hedenquist and Lowenstern 1994; Heinrich 2007a; mixed metamorphic fluids, Trommsdorff and Skippen 1986; Heinrich 2007b; and magmatic fluids, Webster and Mandeville 2007). This has encouraged the idea that certain common natural melt–fluid–gas systems pass from immiscible (with multiple coexisting fluids) to miscible behavior (a single homogeneous fluid) with changing pressure and/or temperature (increasing or decreasing) or changing composition (see recent summary by Veksler 2004). The term supercritical is applied to any solution (liquid or solid) occurring at a temperature or pressure above its critical point. In the case of supercritical fluid this corresponds to conditions where liquid …

High temperature melts, fluids and gases progressively organize themselves structurally during cooling, usually causing separation of solids, liquids or gases. In many different chemical systems this phase separation results in distinct chemical separation ( immiscibility ), with associated contrasting physical properties in the separating phases. Because of the variety in chemistries, and relative changes in entropy and volumes of the natural mixtures compared to the separated phases, immiscibility can occur in different chemistries during heating and compression, as well as during cooling and decompression. Although the main emphasis in this volume is on fluid–fluid equilibria, there are good examples and much literature on liquid–liquid equilibria in synthetic silicate melts and natural magmas. In fact much of our understanding of phase separation (for geochemists generally taken simply as liquid–vapor equilibria) actually originates from liquid–liquid immiscibility studies of silicate melts. For a one-component system there are three distinct regions in PT space where either solid or liquid or vapor exist as the stable phase. These are separated by three univariant curves (i) the solid–liquid curve, the solidus, (ii) the liquid–vapor curve, which can lead to the most commonest form of critical point (see Fig. 1a⇓), and (iii) the solid–vapor curve, which reflects direct vaporization (sublimation) or condensation of solids from gas. The latter is relevant in some industrial processes and to condensation of stars at vacuum pressures. For each added component with solid, liquid and vapor phases, there is the possibility of mixing of each of the phases, or not. In silicates we are used to recognize immiscibility gaps (solvus) among chemically related minerals (e.g., alkali feldspars), which are miscible at high temperatures in the subsolidus region but are immiscible with cooling and undergo phase separation. We are less used to consider immiscibility between two anhydrous melts (silicate–silicate, silicate–carbonate, silicate–oxide, silicate–sulfide, etc.). Figure 1. Pressure …

Cu K-edge X-ray absorption near edge structure (XANES) spectra were recorded from individual synthetic brine fluid inclusions as a function of temperature up to 500 C. The inclusions serve as sample cells for high-temperature spectroscopic studies of aqueous Cu-Cl speciation. Cu{sup +} and Cu{sup 2+} can both be identified from characteristic pre-edge features. Mixed oxidation states can be deconvoluted using linear combinations of Cu{sup +} and Cu{sup 2+} spectra. This work illustrates how complex Cu XANES spectra can be interpreted successfully. Cu{sup 2+} is the stable oxidation state in solution at room temperature and Cu{sup +} at high temperatures. The change in oxidation state with temperature was completely reversible. Cu{sup +} was found to occur exclusively as the linear species [CuCl{sub 2}]{sup -} in solutions containing KCl with Cu:Cl ratios up to 1:6. In the absence of K{sup +}, there is evidence for higher order coordination of Cu{sup +}, in particular the tetrahedral complex [CuCl{sub 4}]{sup 3-}. The importance of such complexes in natural ore-forming fluids is yet to be determined, but may explain the vapor-phase partitioning of Cu as a Cl complex from a Cl-rich brine.

Here we report on a newly developed, large-volume, cold-sealed capsule design for hydrothermal synthesis experiments in a piston-cylinder apparatus that should be useful for the production of synthetic fluid inclusions at pressures and temperatures not previously attained in gas- or fluid-pressurized reaction vessels. The design is adapted for large-volume experiments using a 30 mm internal-diameter pressure vessel, but can be scaled down to suit smaller pressure vessels, e.g., 15.9 mm (5/8″) internal diameter, if required. Calibration experiments show that temperature varies ±5 °C over the length of a 30 mm (length) × 15 mm (diameter) Cu capsule. The design incorporates the thermocouple within the capsule mass to optimize temperature control. Quartz-hosted H2O inclusions were synthesized over a range of conditions. Fluid-inclusion densities are consistent with the nominal experimental conditions, suggesting a friction correction is not required. This approach has several advantages over conventional hydrothermal experimental methods: (1) substantially higher pressures are attainable in piston-cylinder than hydrothermal and gas-media apparatus; (2) cold-sealing capsules avoid potential problems associated with welded capsules, such as solution modification; (3) capsule fluids are readily sampled ex situ; (4) the use of relatively thick-walled capsules minimizes H2-losses during experiments; (5) synthetic fluid inclusions can be used to derive fluid PVTX properties by combining conventional thermometry with analyses of individual fluid inclusions or independent mineral solubility data.